Antibodies

• I’ve heard of them but just what are they?• Plasma Cells of Effector Cells

• Transcription • Translation• Polypeptide / Proteins• Humoral Response: soluble chemical messengers

• Their Ability• Immunoglobulins: Two Types

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Fig. 53.7

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Fig. 53.7a

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Classes of Immunoglobulins

• Five Classes• Two common classes are the IgM (immunoglobulins in mammals)

and Gamma Globulins or IgG .• Provide most of the specific immunity against bacteria and

viruses in extracellular fluid. • IgE are antibodies involved with allergic responses.

• Mast Cells and Histamine

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Antibody Structure

• All immunoglobulins of a particular class have the same amino acid sequence for the “Stem” or “Constant” region.

• Other Constant Regions

• Variable Region

• The Concept of “Domains”• Variable• Joining• Constant

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Fig. 53.7b

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Activation and Secretion

• Upon activation by an antigen to the receptor• So how can the secreted antibodies be different from the antibody that

is in the membrane?• DNA mRNA or pre-mRNA spliced transcription

polypeptide

• Spliceosomes

• Therefore the B cell receptor and the secreted antibodies “differ” but still recognize the same antigen.

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The Unique Region: Variable Region

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RAG: Recombination Activating Gene

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Fig. 53.8-2

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Fig. 53.8-3

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Fig. 53.8-4

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Fig. 53.8-5

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Fig. 53.8-6

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Increasing the Immunoglobulins

• Funny things happen while joining of the variable, heavy chain domain and joining areas • Nucleotides can be lost or clipped off. • This will result in the presence of different amino acid sequences.

• Hypermutation• Occurs in the antigen binding site of the variable region• Point Mutations

• Cytosines and uracils differ by the presence of an amino group. – “Cytosines get deaminated into a uracils leading to C to T

mutations during DNA replication. “ • The presence of these uracils also attracts DNA polymerases

which are not good at repairing DNA (error-prone DNA polymerases).

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In Summary:

• The Three Processes Giving Us the Ability to Recognize So Many Antigens1) Gene Recombination2) Imprecise Joining of Domains3) Hypermutation

• All this is specific to lymphocytes. • And the body makes millions of different lymphocytes.

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Figure 43.9 The interaction of T cells with MHC molecules

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Figure 43.12a The functioning of cytotoxic T cells

MHC I Glycoprotein: all nucleated cells

Tumor Cells: present tumor antigens to Tc cells so the body can fight against tumors

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Humoral immune response• Bacteria penetrate body lining and enter interstitial fluid• Move into lymph ad lymph nodes• Macrophage and a B cell recognize and bind to bacterium• Helper T cell binds to macrophage that phagocytized bacterium and

presents epitopes next to class II MHC on APC• Helper T cell activated (macrophage provides costimulus and

cytokine stimulation)• Stimulated Helper T cell undergoes clonal selection and multiplies• Activated helper T cells stimulate antigen bound B cells to

proliferate and differentiate• Plasma cells secrete antibodies, memory cells wait in reserve for

future invasions

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Fig. 53.11

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Fig. 53.12

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Key Genes • HIV’s env gene encodes surface or envelope proteins

gp120/gp41.

• Gag gene encodes for the matrix and capsid proteins

• pol gene codes for reverse transcriptase, protease and integrase.

• Both gag and pol genes are made into GAG and GAG-POL multiprotein chains that are not yet functional. They assemble at the inner surface of the cell membrane and bud off tgo form an immature viral particle.

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Key Enzymes

• Reverse Transcriptase

• Integrase– LEDGF: molecule that binds to integrase and specific sites on

the cell’s chromosomes helping to target exactly where the viral DNA can insert itself into the host’s genome.

• How could they experimentally determine if LEDGF actually played a role in the integration of the viral DNA?

• Protease– Protease inhibitors

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HIV MOLECULAR BIOLOGY Cells Alive Animation: www.cellsalive.com

1. Retrovirus

2. Several strains of HIV

3. Binding and Entry

a) RNA has 9 different genes encoding 15 proteins.

b) Entry requires the fusion of the viral envelope with the host cell membrane.

c) The viral envelope interacts with specific cell surface receptors.

d) gp120 is on the viral surface binds to the CD4 receptors on the T cells; after binding it changes shape to expose the hydrophobic tips of gp41.

e) gp41: after exposure it will insert into T cell membrane causing fusion of the host T cell and virus.

f) Binding of gp120 and CD4 produces changes in gp41, it springs open and grabs the lipid bilayer of the target cell. This promotes fusion.

4. Virus attacks cells with CD4 receptors on TH cells(immune system cells)

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5. Role of TH cells? Stimulates the immune system; they bind to MHC II APC’s which stimulates both cell mediated immunity and antibody secretion from plasma cells

6. HIV binds to probably several proteins: CD4 receptor, a fusin protein which allows for the envelope of the virus to join with the

T cell and another receptor CCR5 which is on macrophages.7. CXCR4 binds to T-cells in later stages of the disease. 8. In up to 13% of individuals of northern European descent, a

naturally occurring deletion of 32 base pairs in the CCR5 gene results in a mutant CCR5 receptor that never reaches the cell surface.

a) Individuals homozygous for this mutation (1-2% of Caucasian population) are almost completely resistant to HIV infection.

9. Detection for HIV antibodies: 1-12 months after exposure/infection.